FIG. 1 shows an apparatus, disclosed in U.S. patent application Ser. No. 08/384,997, for electrically suppressing the electrochemical potential (ECP) near a BWR component which is susceptible to intergranular stress corrosion cracking (IGSCC). The apparatus is a self-contained means of locally protecting critical portions of metals, such as welds, by suppressing ECP in the immediate vicinity of that portion of the metal requiring protection in operating BWR plants.
The apparatus shown in FIG. 1 is based on the concept of supplying electrons directly and locally to the surface of a sensitized metallic structural member 2, as in the case of the heat affected zone 6 of a weld 4, thereby inhibiting IGSCC. The electrical system depicted in FIG. 1 is capable of supplying sufficient electrons to the metal surface to inhibit the corrosion reaction due to local ECF exceeding the threshold value at which IGSCC can occur.
In the circuit of FIG. 1, the center electrical conductor of a small mineral-insulated steel sheathed cable 16 is attached to the metal surface to be protected against IGSCC and connected to an electrical control circuit 10 that operates off the low-voltage DC power supply 20. The control circuit 10 and DC power supply 20 are enclosed in a housing 8 made of material able to withstand thermal and radiological conditions inside a boiling water reactor, but outside the reactor core. The passive conductor of a twisted-shielded pair of cable conductors is connected to a reference electrode 18 located in the oxidizing coolant near the metal surface and to a terminal of the control circuit. The current collected at the metal surface is controlled by the applied voltage on the load resistor R via an electrical conductor connected to the surface of the metal to be protected and to another terminal of the control circuit. This current I is converted to a voltage drop across R, which is input to a differential amplifier 12 of gain G. The differential amplifier output is the effective voltage "error signal" which is integrated by the operational amplifier 14 with time constant .tau.=R.sub.1 C. The small stand-off resistor R.sub.2 depletes excess charge build-up on the feedback capacitor C to eliminate any possibility of integrator malfunction. The collected current is dissipated in the load resistor R. Electron depletion of the metal and IGSCC are defeated since electrons are forced to flow into the metal to compensate for those that would be lost by oxidation of the metal.
The apparatus shown in FIG. 1 has a power supply 20 which requires no external power source, but rather is energized by electrons (also referred to herein as .beta.-particles) produced during nuclear decay. The source current I.sub.s (see FIG. 1) arises from the collection of nuclear decay electrons and produces a voltage across the source resistor R.sub.s which is a slowly decreasing function of time (because of the emitter decay). The Zener diode 24 and load resistor R.sub.L stabilize and limit the output voltage B.sub.+, since the voltage drop across the diode is essentially the same for all reverse currents I.sub.Z flowing through it in the breakdown region of the device. The voltage B.sub.+ is regulated and stabilized, since large changes in diode current produce small changes in diode voltage. The resulting voltage across the load resistor R.sub.L, due to the load current I.sub.L, is insensitive to the .beta.-emitter decay and can be used to power the active components in the control circuit.
In accordance with the foregoing teaching, the source of electrons was the decay of a radioactive isotope, depicted in FIG. 1 as a current source 22. For ease of handling and fabrication, the proposed isotope was a .beta.-emitter (nuclear electrons) without decay .gamma.-radiation. The following .beta.-emitting isotopes were identified as being suitable candidates: H.sup.3, C.sup.14, Si.sup.32, Sr.sup.90 and Ru.sup.106. The Ru.sup.106 isotope was preferred because of its 368-day half-life and 39.4 keV .beta.-ray. All these isotopes have a single decay mode with no prompt .gamma. emissions.
However, further study of the table of isotopes has revealed that Ru.sup.106 decays to Rh.sup.106, which is both a .beta.-emitter and a .gamma.-emitter in its decay to stable Pd.sup.106. A preferred .beta.-emitter is one with adequate half-life for practical application and with no .gamma.-emission in its decay chain. The avoidance of .gamma.-emission serves to simplify handling and fabrication, and to reduce leakage currents.